Whats A Microcontroller v3

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When you start building your circuit, make sure to check it against the schematic symbol and part drawing. For the part drawing, note that the LED’s leads are different lengths. The longer lead is connected to the LED’s anode, and the shorter lead is connected to its cathode. Also, if you look closely at the LED’s plastic case, it’s mostly round, but there is a small flat spot right near the shorter lead that that tells you it’s the cathode. This really comes in handy if the leads have been clipped to the same length.

LED Test Circuit Parts

(1) LED – Green

(1) Resistor – 470 Ω (yellow-violet-brown)

Identifying the parts: In addition to the part drawings in Figure 2-2 and Figure 2-4, you can use the photo on the last page of the book to help identify the parts in the kit needed for this and all other activities.

Building the LED Test Circuit

You will build a circuit by plugging the LED and resistor leads into small holes called sockets on the prototyping area shown in Figure 2-5. This prototyping area has black sockets along the top and along the left. The black sockets along the top have labels above them: Vdd (+5 V), Vin (the unregulated voltage straight from your battery or power supply), and Vss (0 V, also called ground). These are called the power terminals, and they will be used to supply your circuits with electricity. The black sockets on the left have labels like P0, P1, up through P15. These are sockets that you can use to connect your circuit to the BASIC Stamp module’s input/output pins.

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Direction does matter for the LED, but not for the resistor. If you plug the LED in backward, the LED will not emit light when you connect power. The resistor just resists the flow of current. There is no backwards or forwards for a resistor.

9Reconnect power to your Board of Education or HomeWork Board.

9Check to make sure your green LED is emitting light. It should glow green.

If your green LED does not emit light when you connect power to the board:

9Some LEDs are brightest when viewed from above. Try looking straight down onto the dome part of the LED’s plastic case from above.

9If the room is bright, try turning off some of the lights, or use your hands to cast a shadow on the LED.

If you still do not see any green glow, try these steps:

9Double check to make sure the LED’s cathode and anode are connected properly. If not, simply remove the LED, give it a half-turn, and plug it back in. It will not hurt the LED if you plug it in backwards, it just doesn’t emit light. When you have it plugged in the right direction, it should emit light.

9Double check to make sure you built your circuit exactly as shown in Figure 2-6.

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9If you are using a What’s a Microcontroller kit that somebody used before you, the LED may be damaged, so try a different one.

9If you are in a lab class, check with your instructor.

Still stuck? Try these free online resources:

Visit the Stamps In Class moderated forums: If you don’t have an instructor or friend who can help, you can always check with the Stamps in Class forum at http://forums.parallax.com. If you don’t get your questions answered there, you can contact Parallax Technical Support department by following the Support link at www.parallax.com.

How the LED Test Circuit Works

The Vdd and Vss terminals supply electrical pressure in the same way that a battery would. The Vdd sockets are like the battery’s positive terminal, and the Vss sockets are like the battery’s negative terminal. Figure 2-7 shows how applying electrical pressure to a circuit using a battery causes electrons to flow through it. This flow of electrons is called electric current, or often just current. Electric current is limited by the resistor. This current is what causes the diode to emit light.

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Chemical reactions inside the battery supply the circuit with current. The battery’s negative terminal contains a compound that has molecules with extra electrons (shown Figure 2-7 by minus-signs). The battery’s positive terminal has a chemical compound with molecules that are missing electrons (shown by plus-signs). When an electron leaves a molecule in the negative terminal and travels through the wire, it is called a free electron (also shown by minus-signs). The molecule that lost that extra electron no longer has an extra negative charge; it is now called neutral (shown by an N). When an electron gets to the positive terminal, it joins a molecule that was missing an electron, and now that molecule is neutral too.

Figure 2-8 shows how the flow of electricity through the LED circuit is described using schematic notation. The electrical pressure across the circuit is called voltage. The + and

– signs are used to show the voltage applied to a circuit. The arrow shows the current flowing through the circuit. This arrow is almost always shown pointing the opposite direction of the actual flow of electrons. Benjamin Franklin is credited with not having been aware of electrons when he decided to represent current flow as charge passing from the positive to negative terminal of a circuit. By the time physicists discovered the true nature of electric current, the convention was already well established.

A schematic drawing (like Figure 2-8) is a picture that explains how one or more circuits are connected. Schematics are used by students, electronics hobbyists, electricians, engineers, and just about everybody else who works with circuits.

Appendix B: More about Electricity contains some glossary terms and an activity you can try to get more familiar with measurements of voltage, current and resistance.

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ACTIVITY #2: ON/OFF CONTROL WITH THE BASIC STAMP

In Activity #1, two different circuits were built and tested. One circuit made the LED emit light while the other did not. Figure 2-10 shows how the BASIC Stamp can do the same thing if you connect an LED circuit to one if its I/O pins. In this activity, you will connect the LED circuit to the BASIC Stamp and program it to turn the LED on and off. You will also experiment with programs that make the BASIC Stamp do this at different speeds.

There are two big differences between changing the connection manually and having the BASIC Stamp do it. First, the BASIC Stamp doesn’t have to cut the power to the development board when it changes the LED circuit’s supply from Vdd to Vss. Second, while a human can make that change several times a minute, the BASIC Stamp can do it thousands of times per second!

LED Test Circuit Parts

Same as Activity #1.

Connecting the LED Circuit to the BASIC Stamp

The LED circuit shown in Figure 2-11 is wired almost the same as the circuit in the previous exercise. The difference is that the resistor’s lead that was manually switched between Vdd and Vss is now connected to a BASIC Stamp I/O pin.

9Disconnect power from your Board of Education or HomeWork Board.

9Modify the circuit you were working with in Activity #1 so that it matches Figure 2-11.

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Resistors are essential. Always remember to use a resistor. Without it, too much current will flow through the circuit, and it could damage any number of parts in your circuit, BASIC Stamp, or Board of Education or HomeWork Board.

Turning the LED On/Off with a Program

The example program makes the LED blink on and off one time per second. It introduces several new programming techniques at once. After running it, you will experiment with different parts of the program to better understand how it works.

Example Program: LedOnOff.bs2

9Enter the LedOnOff.bs2 code into the BASIC Stamp Editor.

9Reconnect power to your Board of Education or HomeWork Board.

9Run the program.

9Verify that the LED flashes on and off once per second.

9Disconnect power when you are done with the program.

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that the LED flashes on and off, over and over again. It will keep flashing until you disconnect power, press and hold the Reset button, or until the battery runs out. Code that repeats a set of commands indefinitely is called an infinite loop.

A Diagnostic Test for your Computer

Although it’s not common, there are some computer systems, such as certain laptops and docking stations, that will halt the PBASIC program after the first time through a DO...LOOP. These computers have a non-standard serial port design. By placing a DEBUG command in the program LedOnOff.bs2, the open Debug Terminal prevents this from possibly happening. You will next re-run this program without the DEBUG command to see if your computer has this non-standard serial port problem. It is not likely, but it would be important for you to know.

9Open LedOnOff.bs2.

9Delete the entire DEBUG command.

9Run the modified program while you observe your LED.

If the LED blinks on and off continuously, just as it did when you ran the original program with the DEBUG command, your computer will not have this problem.

If the LED blinked on and off only once and then stopped, you have a computer with a non-standard serial port design. If you disconnect the serial cable from your board and press the Reset button, the BASIC Stamp will run the program properly without freezing. In programs you write yourself, you will always need to add a single DEBUG command, such as:

DEBUG "Program Running!"